I've made a ton of these--I prefer Pellon ShapeFlex--it's a woven, fusible--much like you find in fine tailored menswear. I buy it by 54" roll from a LQS and I like the handiness of that over the 24" width--but harder to find. If I'm having to use those poly woven shirts that have "holes" in them, I prefer the Pellon FW 911 (non-woven) as it backs those holes. Like Feline said, fabric softener makes it harder to fuse and shirts with poly in them are more difficult too as you need a lower heat on the shirts. I've found that if I use a moist pressing cloth on poly shirts it really helps fuse.